Why do we say "loaded to the gills"?

Giuseppe looked at Francisco and sighed, “What are we gonna do?” Francisco was looking hard at their loaded boat. They weren’t going to get anymore cargo into the cargo hold, and they couldn’t lash anything else onto the deck. That meant that they were going to miss out on a lot of profits, and several clients were going to be unhappy. Francisco rubbed his chin. The roughness of a newly sprouted goatee scratched his hand soothingly. “I say we’re gonna go fishin’.” “Fishing? Fishing? We have a boat so filled with stuff that we can’t put anymore on it, and we still have stuff to put on it. And you say we should go fishing?” “Trust me, my brother. Have I ever steered you wrong?” Giuseppe thought back to the cliffs along the coast of England but decided not to bring that up. “You know that I hate when you say that…” “That is why I say it. Now let’s go get our poles and nets.” The two brothers got their poles and were fishing near their ship within minutes. Giuseppe started as soon as he got down to the water. Francisco set up the net so that they could throw their freshly caught fish into it. It was still in the water, so the fish would be able swim around. They just wouldn’t be able to escape the net. Giuseppe caught a huge sea bass. Soon after, Francisco caught a swordfish. Giuseppe caught two more bass and a sword fish. His brother caught a bass, two swordfish and a boot. He threw the boot onto the shore; he would clean it up later. When they had finally caught two dozen assorted fish, Francisco pulled his pole out of the water. “Now, we will solve our transport problem. Come and help me Giuseppe.” Giuseppe had no idea what his brother was up to, but he followed him to where they had piled the excess cargo. Francisco grabbed several of the watertight satchels and a coil of rope. He then walked back to where the fish were swimming in the net. The first thing Francisco did was to grab a fish and tie a rope to it. He tied the other end of the rope to the ship. He then loaded several of the satchels onto the fish and tied them down – careful not to cover their eyes or gills. Giuseppe followed his brother’s lead. After tying of his fifth fish, Giuseppe noticed that the first fish he had loaded had died. Then the second and third fishes died. “Hey, Francisco, this idea of yours – it is crazy. The fish just keep dying.” Francisco looked at his fish. They seemed to be okay. He looked at Giuseppe’s fish and understood the problem immediately. He corrected his brother and they finished loading up the rest of the fish with the rest of their cargo. As they sailed into their hometown, Francisco patted his brother on the back. “You see, Giuseppe, all of the fish are still alive, and our cargo has made it home safe.” “But why did those first fish die, Francisco?” Francisco thought about it for a bit and decided that his brother might understand the explanation. “When you loaded your fish, you gave them too much stuff to carry. They couldn’t breathe…” His brother interrupted, “That’s why we loaded them to the gills!”